Water based plasticizer free polyurethane-wax coating and repair composition and method

ABSTRACT

A coating composition comprising a colloid of a thermoplastic, elastomeric polyurethane and a wax dispersible in water is applied to the surface of a substrate, drying rapidly to form a tough, durable, protective film bonded firmly to the surface.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.08/905,828, filed Aug. 4, 1997, entitled Water Based Plasticizer FreePolyurethane-Wax Coating & Repair Composition & Method, now U.S. Pat.No. 6,001,906, which is incorporated herein by reference and made a partof this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a non-toxic, water-based coating compositionwhich, when applied to the surface of a substrate, particularly leatherand vinyl materials, dries rapidly to provide a tough, protective filmbonded firmly to the surface. This invention also relates to the methodof using this coating composition.

2. Background Discussion

It is desirable to apply protective coatings to a wide variety ofmaterials. For example, vinyl or leather upholstery for furniture andautomobiles, articles of clothing like shoes and jackets, and objectssuch as purses, wallets, etc. all have an enhanced appearance if coatedwith a liquid which forms a durable film after evaporation of thecarrying agent for the film forming material. U.S. Pat. No. 3,597,373illustrates a typical coating composition which uses hydrocarbonsolvents for the film forming material. These solvents are irritating toskin and mucus membranes, or even toxic upon prolonged exposure. Anotherdisadvantage of hydrocarbon solvent based resin coatings is that thefilm formed tends to be inflexible. To overcome this, plasticizers areblended in the coating. These plasticizers migrate from the film over aperiod of time and may leave on the surface of the film a sticky, oilyresidue. Expensive blocking agents are used to inhibit the migration ofthe plasticizers from the film'surface. Eventually, the film becomesbrittle when the plasticizers have volatized or have been transferreddue to contact with other substrates and are no longer present in asufficient amount to maintain the desired flexibility. Antioxidants areused in hydrocarbon solvent based resin coatings to speed drying andprevent decomposition of the film by air oxidation. Stabilizers are alsoused in hydrocarbon solvent based resin coatings to impart ultravioletlight resistance to both the coating and the resulting film. These areexpensive and sometimes toxic ingredients.

SUMMARY OF THE INVENTION

This invention provides a coating composition which uses water as thecarrying agent for the film forming material, and thereby avoids theproblems associated with hydrocarbon based coatings. Moreover, a resin,or mixture of resins, is used as the film forming material which has theproper flexibility and abrasion resistance for the application withoutthe need for inclusion of plasticizers into the coating. It has beenfound that a mixture of resins is desirable in some circumstances totailor the coating composition to the particular application. Forexample, a coating applied to vinyl automobile upholstery must be moredurable and tougher than a coating for hand bags and gloves.

The coating composition of this invention has several features, nosingle one of which is solely responsible for its desirable attributes.Without limiting the scope of this invention as expressed by the claimswhich follow, its more prominent features will now be discussed briefly.After considering this discussion, and particularly after reading thesection entitled, “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS,”one will understand how the features of this invention provide itsadvantages, which include high impact and abrasion resistance, excellentelongation and flexibility, outstanding stain and chemical resistance,early film integrity, fast dry-to-handle properties, air drying,outstanding toughness, excellent adhesion to heat and solvent sensitivesubstrates, compatible with anionic pigments and dispersing agents,water thinnable, suitable for white and light pastel shades, usable asprimer and topcoat, stable to repeated freeze/thaw cycles, low solventcontent, non-flammable, water clean-up, non-toxic, creates a high heatresistance barrier, high gloss, if desired, and is safe to use.

Briefly, this invention is a non-toxic, water-based, colloidalcomposition including a thermoplastic, elastomeric polyurethane and waxdispersed in water. After evaporation of the water carrying agent, thepolyurethane and wax combine to form a tough film bonded to the surfaceof the substrate to which the coating composition is applied. Thecoating, in some instances, is absorbed by leather substrates, and thistends to improved the bond between the film and the substrate.

The composition may clear or pigmented, that is transparent ortranslucent or opaque. A major portion of solids present in thecomposition are polyurethane and a minor portion of solids present inthe composition are wax. The polyurethane is the principal film formingingredient, and the wax adds mar resistance to the film. It is believedthat the polyurethane and wax interact synergistically to provide abarrier for a substrate which unexpectedly has a high heat and abrasionresistance and, high gloss, when desired. The film formed on, forexample, vinyl or leather, creates a high heat resistant barrier. Thefilm without flattening agents has a high gloss. This combination ofpolyurethane and wax is mainly responsible for the desirable attributesof this invention.

The composition is applied as a coating to the surface of a substrate,and dries rapidly in air at ambient temperatures and pressures, fillingany cracks in the surface, to form a tough, durable film that is bondedfirmly to the surface. At cold ambient temperatures or high humidityconditions, hot air may be used to speed the rate of drying. It may beused to coat such materials as, for example, leather, vinyl polymers,flexible and semi-rigid plastics, rubber, polycarbonate,reaction-injection molding compound, thermoplastic polyurethane,acrylonitrile-butadient-styrene, wood, aluminum, and galvanized metal.

A dispersing agent, a water soluble leveling agent, and a defoamingagent may also be present in the composition. The dispersing agent ispresent in the composition in an amount ranging between about I andabout 30 weight percent. The dispersing agent insures that thepolyurethane is uniformly distributed as tiny, solid particles whichremain in suspension. The preferred dispersing agent isN-methylpyrrolidone. The ratio of water to dispersing agent in thecomposition is from 20/80 to 10/90 by volume. The leveling agent is asurfactant which insures that the film formed upon drying is integral,uniform, and substantially free of discontinuities such as pictureframing, edgepull, pin holes, cratering, and fish eyes. The levelingagent is present in an amount ranging from about 0.05 to about 6 weightpercent. One suitable leveling agent is a mixture of polyfunctionalcomplex organic compounds in 2-butoxy ethanol. The defoaming agent stopsthe formation of bubbles during blending of ingredients that wouldhamper the formation of the desired uniform, integral film. Thedefoaming agent is present in an amount sufficient to suppress foaming,typically in an amount ranging from 0.05 to 6 weight percent.

A polyurethane colloid is used as the principal ingredient of thecomposition of this invention. One of the desirable features of thepolyurethane used in this invention is that it forms a flexible film.This imparts pliability to the substrate, tending to restore itsoriginal hand. Hand is the aesthetic feel of the surface of thesubstrate. The coating of this invention produces a treated substratethat has a hand which approaches the original, untreated substrate. Acharacteristic of the polyurethane film is that its has a percentelongation in the range between about 150 and about 700.

The polyurethane used is a fully reacted polyurethane elastomer with nofree isocynate, is hydrolytically stable, and when completely dry, doesnot redissolve in water. The polyurethane is preferably present in thecomposition in an amount ranging from 20 to 55 weight percent.Preferably, it is an aliphatic polymer having a average molecularweight, calculated on the basis of the gel permeation chromatographicmethod, ranging from 50,000 to 100,000. Although an aromatic polymer maybe used, the film tends to yellow or darken over time, and consequently,an aromatic polymer is unsuitable when a clear, transparent film isdesired. The particles of polyurethane vary widely in size, but themajor portion range between about 200 and about 800 microns.

In accordance with one feature of this invention, the coatingcomposition is designed to be more rougher or more flexible depending inthe intended application. A blend of a base polyurethane having anelongation of approximately 400 percent is mixed with either (1) arelatively hard polyurethane having an elongation of approximately 150percent or (2) a relatively soft polyurethane having an elongation ofapproximately 700 percent depending on the application. The basepolyurethane alone was found to be suitable for many applications, butwas not tough enough when used, for example, with automobile vinyl orleather upholstery or was not flexible enough when used on, for example,clothing. To make the film tougher, a blend comprising of this basepolyurethane and the less flexible but tougher polyurethane was used.Conversely, to make the film more flexible, a blend comprising of thisbase polyurethane and the more flexible but less tough polyurethane wasused.

The wax is dispersed in water and then this dispersion is blended withthe polyurethane colloid. The particles of wax vary widely in size, butthe major portion range between about 10 and about 50 microns.Preferably, a wax emulsion is employed. The wax comprises from 20 to 40weight percent of the aqueous dispersion or emulsion. The co-solvantsmay be used to reduce or extend the drying times. After blending withthe polyurethane colloid to form the composition of this invention, thewax is present in the composition in an amount ranging from about 1 toabout 5 weight percent, and comprises between about 0.5 and 4 weightpercent of the total solids. The wax has a softening point rangingbetween about 180 to about 290 F, and a hardness ranging typicallybetween about 0.5 and about 4.0, according to American Standard TestingMaterials Procedure ASTM-D5. Very hard waxes, for example waxes with ahardness of about 0.5, significantly reduce gloss. Surprisingly, suchwaxes when used in the composition of this invention do not impairgloss, and the film formed upon evaporation of the water provides a highgloss appearance. A suitable wax is a polyethylene having a averagemolecular weight, calculated on the basis of the gel permeationchromatographic method, ranging between about 6000 and about 11,000 anda pH of about 9 to about 11. Other suitable waxes are halogenatedpolyolefins.

Other ingredients may include co-solvents, flattening agents, rheologycontrol compounds, mar resistant additives such as a silicone polymer ora fluorocarbon polymer, acrylic resins, and dyes or pigments. Theco-solvents extend drying times, which may be desirable in someapplications, and improve substrate wetting. Suitable co-solvents are,for example, short chain ketones such as acetone which will reduce thedry time, N-methypyrrolidone, ethylene glycol, and propylene glyol. Theflattening agents may be used to diminish the glossy appearance of thefilm, and preferably are colloidal silicas or platey talc. The rheologycontrol compounds improve lay down of the film and the dispersion of thedyes and color pigments so that settling is reduced. A suitable rheologycontrol compound is ethyl acrylate acrylic emulsion copolymer. Therheology control compound has a high solids content typically from about30 to about 50 weight percent and is on the acidic side having a pHtypically ranging from 2.2 to 3.5. Water borne acrylic resin may beadded to reduce the cost. In accordance with one feature of thisinvention, the composition is essentially free of plasticizers andantioxidants as separate additives.

If the film is exposed for prolonged periods of time to sun light, itmay be desirable to blend with the composition ultraviolet lightstabilizers. In many situations, however, such ultraviolet lightstabilizers are unnecessary. Therefore, the composition may also beessentially free of ultraviolet light stabilizers as separate additives.

This invention includes a method of protecting the surface of asubstrate, comprising:

(1) applying to the surface a thin coating of the water-based, colloidalcoating composition, and

(2) drying the coating to evaporate the water, so that a tough, durablefilm is formed on the surface which is bonded firmly to said surface.

This invention also includes the product coated with the composition ofthis invention. Such product comprises a substrate having a surfacecovered by a solid, thin barrier which adheres firmly to the surface andis formed upon evaporation of water after the application of anon-toxic, water-based, colloidal coating composition including athermoplastic, elastomeric polyurethane and wax dispersible in water.The film has a thickness ranging between about 5 and about 100 microns.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following lists the main ingredients of the preferred coatingcomposition of this invention, and their sources.

I. RESIN COLLOID

The resin colloid is a dispersion of polyurethane in water usingN-methylpyrrolidone as the dispersing agent. The best materialdiscovered for use in the coating composition of this invention isSPENSOL® L-53 Resin (Reichold Chemicals, Inc). It is an aliphatic,anionic, colloidal, water dispersion of thermoplastic urethane polymersusing N-methylpyrrolidone. The polyurethane is present as solidparticles suspended in water, with the following particle sizes:

10% less than 190 microns

5% less than 390 microns

90% less than 810 microns

Typical Resin Colloid Analysis Percent solids by weight 30.00  Viscosityat 25 C. 0.50 Stokes Specific Gravity 1.04 Pounds/Gal Solution 8.70Pounds/Gal Solids 9.70 pH 8.00 Solvents Water/NMP 82/18

II. WAX & ADDITIVES

The preferred wax is a polyethylene wax which may be used alone or incombination with silicone or fluorochemical surfactants. The followingare suitable waxes:

1. Wax Dispersions such as:

a. polyethylene wax UCD 21160 (Universal Color Dispersions) a divisionof Morton Chemicals, Inc.

b. polyethylene and synthetic waxes SLIP-AYD® SL Series (Daniel ProductsCo.)

c. 2707 Wax polyethylene wax (RBH Dispersions Inc.)

2. Wax Emulsions such as:

a. Michem Polymer Emulsions (Various types available from MichelmanChemical Inc.)

b. Jonwax 26 & 120 (Johnson Wax—S. C. Johnson & Son)

3. Powdered Wax such as:

a. S-681, polyethylene, S-368 polyethylene, and SST-3, PTEF (Teflon),(Shamrock Chemical Corp.)

b. MP-22XF, synthetic paraffin, Aquapoly blend of PTFE and hydrocarbon250, and Polysilk-14, a blend of fluorocarbon and hydrocarbon powders(Micro Powders, Inc.)

4. Mar Resistant Silicone & Fluorochemical additives such as:

a. a polyether modified dimethylpolysiloxane copolymer BYK® 301,306,321(Byk Chemie, USA)

b. a nonionic fluorosurfactant of the polyoxythylene type LODYNE™ S-107B(Ciba-Geigy Corp.)

c. FC-129 and FC-430 (3M Corp.)

III. FLOW CONTROL AGENTS

The flow control agents are surfactants such as:

1. a dioctyl sodium sulfo-succinate in petroleum distillate AEROSOL®OT-S (American Cyanamid Co.)

2. a mixture of 2,4,7,9-tetra methyl-5-decyne-4,7-diol and ethyleneglycol SURFYNOL® 104 Series (Air Products & Chemicals, Inc.)

3. a polyether modified dimethylpolysiloxane copolymer BYK® Byk301,306,321 (Byk-Chemie, U.S.A.)

4. a nonionic fluorosurfactant of the polyoxythylene type LODYNE™ S-107B(Ciba Geigy Corp.)

5. a fluorochemical with a hydrocarbon tail and a solubilizing groupattached to the tail FLUORAD™ FC-129 and FC-430 (3M Corp.)

6. propylene glycol methyl acetate in a solvent FCABA (Paint Chemicals,Inc.)

7. glycol ether proprietary blend WRACLA (Paint Chemicals, Inc.)

8. WRAPA (Paint Chemicals, Inc.)

IV. DEFOAMERS

1. Foam Master 111, NXZ and VL (Henkel Corp/Coatings Chemicals)

2. Drewplus L447, 464 and 405 (Drew Industrial Div. Ashland ChemicalCo.)

3. Nalco 2300, 2309 and 2343 (Nalco Chemical).

4. Colloid 679,999 and 639AA (Colloids, Inc.)

5. an antifoam/defoamer DEE FO™ PI-3 (Ultra Additives, Inc.)

6. a silicone-free foam suppressor DAPRO™ DF 900, DF911 (Daniel ProductsCo.)

V. PRE-DISPERSED COLORS

Suitable coloring agents are any water thinnable pigment concentrates,for example, the acrylic based color concentrates such as:

1. Sup-R-Conc L Series (Hilton-Davis Chemical Co.)

2. 888 (Colortrend) Series (HULS, Inc.)

3. Aurasperse

VI. RHEOLOGY CONTROL COMPOUNDS

1. a hydroxyethyl cellulose CELLUSIZE™ QP4400 and QP-1500 (Union CarbideCorp)

2. an acrylic alkali soluble emulsion ACRYSOL™ GS, RM5, ASE-60 andASE-95, QR-708 (Rohm & Haas Co.)

3. Bentone EW and LT, clays, (Rheox Inc.)

4. POLYOX™ WSR, polyethylene oxide Series (Union Carbide Corp.)

5. an in-situ thickener ALCOGUM® L37, sodium polyacrylate (AlcoChemical)

6. UCAR SCT-200, SCT 270 proprietary (Union Carbide)

EXAMPLE

A typical coating composition of this invention is made utilizing 93.28gallons of the resin colloid SPENSOL® L-53 made by Reichold ChemicalsCompany. To this is blended 3.34 gallons of the polyethylene waxdispersion UCD 21160 made by Morton Chemicals. The mixture is stirredvigorously to completely mix the wax dispersion with the colloid. Themixture is diluted with 1.55 gallons of water, and to this mixture isadded 0.83 gallons of the defoamer DAPRO™ DF 900 (Daniel Products Co.),0.25 gallons of the flow control agent FCABA (Pain Chemicals, Inc.) and0.50 gallons of the flow control agent WRACLA (Pain Chemicals, Inc.),and 0.25 gallons of the rheology control compound ALCOGUM® L-37 (AlcoChemicals). The mixing of the ingredients is conducted at ambienttemperature and pressure.

Preparation of Leather or Vinyl Material

The surface of the substrate material should be free of all silicones,waxes, grease, oil, or dirt which would prevent or impair the coatingcomposition from forming the desired tough, durable protective film onthe surface of the leather and vinyl material. Conventional solvents anddegreasing detergents may be used to prepare surface for proper wettingby the coating composition. In some cases where the surface has heavycoating of contaminates, 0000/steel wool may be used in conjunction witha solvent or degreaser. The surface should be wiped clean beforeapplication of the coating composition, and the solvents or degreasersmust not leave any residue. Minor surface damage consisting of smallcuts, holes, nicks, etc. may be filled with small amounts of the coatingcomposition and allowed to dry to the touch. If the cut has penetratedthe surface, for example creating a flap, then the flap should be glueddown using the coating composition, pressing the flap in place until thecomposition has set. The composition preferably is applied by spray,brush or sponge foam applicator. A roller, rubber or plastic spreadapplicator may leave undesirable surface texture. The coatingcomposition should be applied in one direction to avoid re-wetting thesemi-dried coating and causing surface imperfections. When dry to thetouch, additional coatings can be applied in other directions to providebetter coverage and a more uniform protective film on the surface.

Application of Coating

One easy way to use the coating composition, for instance, is to dip asponge (No. 1 Latex) in the composition of the EXAMPLE an apply over thesurface of, for example leather, as a thin film. The coating preferablyshould be laid down in a single pass. If additional coats are desiredthe previous coating should be allowed to air dry, which ordinarilytakes only from about 2 to 60 minutes depending on ambient conditions.The evaporation of the water results in the formation of a tough,durable film which adheres to the surface of the substrate. Typicalproperties of this film are:

Film Properties Tack-free Time (minutes) 130 Dry-Hard Time (minutes) 200Sward hardness one day/7 days 10/18 Modulus (psi) 100% 1000  TensileStrength (psi) 5600  Tear Strength (ph) 250 Percent Elongation 400Abrasion Resistance  15 (Taber) CS17 Wheels 1000 Gram load (cycles)1000  Shore Hardness (AID) 85/40

CHARACTERISTICS OF FORMULATION

The film's flexibility may be modified to make the film stiffer orsofter by blending different types of polyurethanes. The higher thepercent elongation of the polyurethane film, the softer the filmcovering the substrate, and vice versa. The following illustrates rangesof ingredients which are typically used in preparing coatings whichimpart different hand characteristics to the substrate. Starting withthe polyurethane colloid SPENSOL® L53 (Reichold Chemicals) variousadditives as listed below may be added to create coatings which providefilms having different hand characteristics for different applications.For example, it would be desirable to have a soft hand when coatingglove leather, and a stiffer hand when coating automobile upholstery.

Minimum/ Maximum Ranges Ingredient (wt %) Characteristics 1. Water 0 to5% Reduces viscosity at higher (Tap) levels and requires additionaladditives due to increase surface tension. 2. Polyethylene Wax 0.25 to6% Lower levels will improve flexibility with loss of mar and abrasionresistance. Higher levels will cause loss of flexibility and adhesion.3. Defoamer 0.25 to 6% DAPRO ™ DF-900 (Daniel Products) Lower levelswill not reduce foam. Higher levels will cause loss of adhesion and filmintegrity. 4. Flow Control Agent 0.25 to 6% FCABA (Paint Chemicals) Flowcontrol will not be enhanced at low levels. High levels will cause lossof adhesion and film integrity. 5. Leveling Agent 0.25 to 6% WRACLA(Paint Chemical) Anti-cratering leveling agent will not be enhanced atlow levels. High levels will cause loss of adhesion and film integrity.6. Rheology Control 0.00 to 10% ALCOGUM ® L37 (Alco Agent (Chemicals)Low levels will allow pigments to settle. High levels will cause coatingto gel. 7. Co-solvents 0.00 to 6% Needed only to improve substratewetting and to adjust evaporation rate, if necessary. 8. Flatting Agents0.00 to 15% For gloss control. Higher levels will affect flexibility,adhesion and film integrity. 9. Pigments As needed to provide coloring.10. Acrylic Resins 0.00 to 40% RHOPLEX ™-MU23, MU-9 (Rohm & Haas).NEOCROYL ™ 601 (Polyvinyl Chemical). CARBOSET ™ 514H (B.F. Goodrich)AROLON ™ 820-W-491820-W-39 (Richold Chemicals) Used for cost savings andpossible adhesion enhancement. Under some conditions may requireadjustments of additives to compensate for reduced flexibility. 11.Polyurethane Blend 0.5 to 25% SPENSOL ® L55 or L56 (Reichold Chemical)Can be used to modify L-53 to increase flexibility and improve softness(hand) where abrasion and mar resistance is not a major factor.

Repeated testing has demonstrated that there is more than one preferredformula and that a range of resin mixes must be used depending on thetype of substrate and its application. For example, when refinishingheavy wear areas, it is advantageous to add a harder polymer to thestandard coating which would be used in the majority of applications.The addition of the polymer will enhance the durability of the coatingby increasing the slip and help in preventing tearing and delaminationdue to hard impact and repeated contact at, or wear on, the same spot.

The addition of acetone (dimethyl ketone) will enhance bonding andpenetration of the substrate, especially of non-porous surfaces. Whenrefinishing extremely flexible substrates, an addition of a highlyflexible resin will help in maintaining long term flexibility and soft“hand”. Experimentation has determined that a particular coating'smaximum performance is a function of usage and type of substrate.Examples of three types of formulations that exhibit desired results inthe following applications that exhibit desired results in the followingapplications:

Automotive leather upholstery SPENSOL ® L-53 74.5 gallons SPENSOL ® L-5120.0 gallons Polyethylene wax 3.0 gallons Water 1.0 gallons DAPRO ™DF-900 0.8 gallons WRACLA 0.5 gallons ALCOGUM ® L-37 0.2 gallons 100.0gallons Household Furniture SPENSOL ® L-53 94.5 gallons Polyethylene wax3.0 gallons Water 1.0 gallons DAPRO ™ DF-900 0.8 gallons WRACLA 0.5gallons ALCOGUM ® L-37 0.2 gallons 100.0 gallons Clothing, Shoes andAccessories SPENSOL ® L-53 55.0 gallons SPENSOL ® L-56 40.0 gallonsPolyethylene wax 2.5 gallons Water 1.0 gallons DAPRO ™ DF-900 0.8gallons WRACLA 0.5 gallons ALCOGUM ® L-37 0.2 gallons 100.0 gallons

SCOPE OF THE INVENTION

The above presents a description of the best mode contemplated ofcarrying out the present invention, and of the manner and process ofmaking and using it, in such full, clear, concise, and exact terms as toenable any person skilled in the art to which it pertains to make anduse this invention. This invention is, however, susceptible tomodifications and alternate constructions from that discussed abovewhich are fully equivalent. Consequently, it is not the intention tolimit this invention to the particular embodiment disclosed. On thecontrary, the intention is to cover all modifications and alternateconstructions coming within the spirit and scope of the invention asgenerally expressed by the following claims, which particularly pointout and distinctly claim the subject matter of the invention:

What is claimed is:
 1. A product comprising a substrate having a surfacecovered by a solid, thin film which adheres firmly to the surface and isformed upon evaporation of water after the application of a non-toxic,water-based, colloidal coating composition which is essentially free ofplasticizers as separate additives, said film consisting essentially ofa wax having a softening point between 180° and 290° F. and athermoplastic, elastomeric polyurethane which is a blend of at lest twopolyurethanes including a base polyurethane having an elongation ofapproximately 400 percent mixed with either (1) a relatively hardpolyurethane having an elongation of approximately 150 percent or (2) arelatively soft polyurethane having an elongation of approximately 700percent depending on the application, said film having a thicknessranging between 5 and 100 microns.
 2. A coating composition which isessentially free of plasticizers as separate additives and consistsessentially of from 20 to 55 weight percent of a thermoplastic,elastomeric polyurethane, from 1 to 5 weight percent of a waxdispersible in water and having a softening point between 180 and 290 F,from 1 to 30 weight percent of a dispersing agent, and the balance waterwhere the ratio of water to dispersing agent is from 20/80 to 10/90 byvolume, said polyurethane being a blend of at least two polyurethaneswith substantially different elongations.
 3. A method of protecting thesurface of a substrate, comprising: applying to said surface a thincoating of a water based, colloidal coating composition which isessentially free of plasticizers as separate additives, and drying thecoating to evaporate the water, so that a film is formed on the surfacewhich is bonded firmly to said surface and has a thickness rangingbetween 5 and 100 microns, said film consisting essentially of athermoplastic, elastomeric polyurethane and a wax dispersible in waterhaving a softening point between 180 and 290 F, said elastomericpolyurethane being a blend of at least two polyurethanes withsubstantially different elongations.
 4. The method of claim 3 where theblend of at least two polyurethanes includes a base polyurethane havingan elongation of approximately 400 percent mixed with either (1) arelatively hard polyurethane having an elongation of approximately 150percent or (2) a relatively soft polyurethane having an elongation ofapproximately 700 percent depending on the application.